The present invention relates to an electric power steering apparatus for detecting a steering torque applied to a steering wheel and controlling driving of a steering assist motor according to the detected steering torque.
An electric power steering apparatus, which controls driving of a steering assist motor based on a result of detection of steering torque applied to a steering wheel and assists power required for steering an automobile by the torque of the motor so as to give a comfortable steering feel to a driver, has spread.
FIG. 1 is a block diagram showing an example of the constitution of xe2x80x9ca power steering apparatusxe2x80x9d proposed by the assignee of the present application in Japanese Patent No. 2694213.
In this power steering apparatus as an electric power steering apparatus, an output signal of a torque sensor 6 is inputted to a control unit 7. The control unit 7 supplies a drive signal to a steering assist motor 8 based on this output signal. The steering assist motor 8 transmits its torque to the lower shaft of a steering shaft.
The torque sensor 6 detects, as the steering torque applied to a steering wheel, a relative displacement in a circumferential direction which is generated between the upper and lower shafts of the steering shaft turning about the axis of the steering wheel according to turn of the steering wheel with twist of a torsion bar interposed between the upper and lower shafts.
A rotation detector 17 for detecting a rotational position of the motor 8 is attached to the rotation shaft of the motor 8.
The control unit 7 is supplied with a rotation detection signal of the rotation detector 17 and an output signal of a vehicle speed detector 18 for detecting a vehicle speed as well as the above-mentioned output signal of the torque sensor 6, and outputs a drive signal for driving the motor 8 and an electromagnetic clutch 16.
A torque detection signal outputted by the torque sensor 6 is supplied to each of a phase compensating unit 71a for stabilizing a system by advancing the phase of the torque detection signal, an angular acceleration calculating unit 71b for calculating an angular acceleration xcfx89xe2x80x2 of turn of the steering wheel, a middle point determining unit 71c for determining the middle point of the steering wheel of a vehicle traveling in a straight direction, and a lock detecting unit 71f for detecting whether the motor 8 is locked.
Moreover, a vehicle speed detection signal outputted by the vehicle speed detector 18 is supplied to each of the lock detecting unit 71f, middle point determining unit 71c, directing current function unit 73a, correction current function unit 73b and change current function unit 73c. 
The directing current function unit 73a is supplied with an output signal of the phase compensating unit 71a and a later-described change current Ia, determines a directing current I for the motor 8 based on the steering torque, change current Ia and vehicle speed detection signal, and outputs the determined directing current I.
The correction current function unit 73b is supplied with the angular acceleration xcfx89xe2x80x2 of the steering wheel outputted by the angular acceleration calculating unit 71b, determines a correction current Ic for correcting the force of inertia of the motor 8 during acceleration and deceleration and the force of inertia of the traveling system of the vehicle based on the angular acceleration xcfx89xe2x80x2 and vehicle speed detection signal, and outputs the determined correction current Ic.
The change current function unit 73c is supplied with a steering angle xcex8 outputted from a later-described steering angle determining unit 71d, determines a change current Ia for changing the characteristic of the directing current I based on the steering angle xcex8 and vehicle speed detection signal, and outputs the determined change current Ia.
The rotation detection signal of the rotation detector 17 is supplied to the lock detecting unit 7 If, middle point determining unit 71c, angular acceleration calculating unit 71b and steering angle determining unit 71d. 
The steering angle determining unit 71d determines a steering angle xcex8 based on the rotation detection signal and the middle point determined by the middle point determining unit 71c, and outputs the determined steering angle xcex8.
The lock detecting unit 71f detects whether the motor 8 is locked, based on the rotation detection signal, when the supplied torque detection signal and vehicle speed detection signal are larger than their predetermined values, respectively. When the lock detecting unit 71f detects that the motor 8 is locked, a driving circuit 72b disconnects the electromagnetic clutch 16.
The output signal of the directing current function unit 73a and the output signal of the correction current function unit 73b are added together by an adder 74a, and the result of addition is supplied to an error amplifier 74b. The error amplifier 74b calculates the difference between the result of addition and a feedback signal from a current detecting circuit 71e for detecting a current flowing in the motor 8, and a voltage corresponding to the calculation result is supplied to the motor 8 by a PWM (Pulse Width Modulation) driving circuit 72a. 
The following description will explain an operation of an electric power steering apparatus having such structures.
The directing current function unit 73a determines a directing current I for the motor 8 based on the torque detection signal whose phase has been compensated by the phase compensating unit 71a, the change current Ia outputted by the change current function unit 73c and the vehicle speed detection signal outputted by the vehicle speed detector 18, and outputs the determined directing current I.
Meanwhile, the change current function unit 73c determines a change current Ia for changing the characteristic of the directing current I, based on the steering angle xcex8 given by the steering angle determining unit 71d and the vehicle speed detection signal outputted by the vehicle speed detector 18, and outputs the determined change current Ia.
The angular acceleration calculating unit 71b reads a torque detection signal T supplied from the torque sensor 6 and a motor rotational speed xcfx89m of the motor 8 supplied from the rotation detector 17, and calculates the angular acceleration xcfx89xe2x80x2 of the steering wheel according to the following equation.
xcfx89xe2x80x2=Txe2x80x3/K+xcfx89mxe2x80x2/nxe2x80x83xe2x80x83(1) 
Here, equation (1) can be given by the following equations.
T=K(xcex8ixe2x88x92xcex8o) 
where
xcex8i: a steering angle of the upper shaft of the steering shaft,
xcex8o: a steering angle of the lower shaft of the steering shaft,
K: a spring constant of the torsion bar.
xcex8ixe2x88x92xcex8o=T/K 
xcex8ixe2x80x3xe2x88x92xcex8oxe2x80x3Txe2x80x3/K 
xcex8ixe2x80x3=Txe2x80x3K+xcex8oxe2x80x3xe2x80x83xe2x80x83(2) 
Meanwhile,
xcex8oxe2x80x2=xcfx89m/n 
xcex8oxe2x80x3=xcfx89mxe2x80x2/n 
(n: the deceleration ratio of the motor rotation shaft to the lower shaft)
∴ According to equation (2),
xcex8ixe2x80x3=xcfx89xe2x80x2=Txe2x80x3/K+xcfx89mxe2x80x2/nxe2x80x83xe2x80x83(1) 
The correction current function unit 73b calculates a correction current Ic for correcting the effects of the force of inertia of the motor 8 and the force of inertia of the traveling system of the vehicle, based on the angular acceleration xcfx89xe2x80x2 calculated by the angular acceleration calculating unit 71b and a vehicle speed detection signal V outputted by the vehicle speed detector 18.
The adder 74a adds the correction current Ic calculated by the correction current function unit 73b to the directing current I calculated by the directing current function unit 73a, and outputs the resultant current. Hence, it is possible to improve a feel of steering.
The error amplifier 74b calculates the difference between the result of addition outputted by the adder 74a and a current flowing in the motor 8 detected by the current detecting circuit 71e, and supplies a voltage corresponding to the calculation result to the PWM driving circuit 72a. The PWM driving circuit 72a controls driving of the motor 8 according to the supplied voltage.
In the above-described electric power steering apparatus, the effects of the force of inertia of the motor 8 and the force of inertia of the traveling system of the vehicle are compensated using the motor rotational speed xcfx89m of the motor 8 supplied from the rotation detector 17, and thus there is a problem that the cost of the parts of the rotation detector 17 is required.
Moreover, although not shown in the drawings, friction compensation of the steering system is performed by controlling a motor current flowing in the motor 8 based on the motor rotational speed xcfx89m of the motor 8 detected by the rotation detector 17, but the torsion bar of the torque sensor 6 twists and the motor 8 does not rotate from the start of steering made by the driver until the friction compensation overcomes the friction torque of the motor 8 and reduction gear. Therefore, friction compensation is not carried out at the start of steering and control of friction compensation is started slightly later, resulting in a problem that the effect of friction compensation is small.
An object of the present invention is to provide an electric power steering apparatus capable of solving the above problems.
An electric power steering apparatus according to the first invention, being provided with a torque detector for detecting a steering torque applied to a steering wheel and a steering assist motor which is driven under control based on a drive current corresponding to the steering torque detected by the torque detector, is characterized by comprising: an angle detector for detecting a turn angle of the steering wheel; angular velocity calculating means for calculating an angular velocity of turn of the steering wheel from the turn angle detected by the angle detector; friction compensating means for calculating an angular acceleration of turn of the steering wheel for friction compensation from the angular velocity of turn calculated by the angular velocity calculating means; and adding means for adding a current related to the angular velocity of turn and a current related to the angular acceleration of turn calculated by the friction compensating means to the drive current.
In this electric power steering apparatus, the torque detector detects a steering torque applied to the steering wheel, and driving of the steering assist motor is controlled based on a drive current corresponding to the detected steering torque. The angle detector detects a turn angle of the steering wheel, and the angular velocity calculating means calculates an angular velocity of turn of the steering wheel from the detected turn angle. The friction compensating means calculates angular acceleration of turn of the steering wheel for friction compensation from the angular velocity of turn calculated by the angular velocity calculating means, and the adding means adds a current related to the angular velocity of turn and a current related to the angular acceleration of turn calculated by the friction compensating means to the drive current.
Accordingly, friction compensation is performed at the start of steering and more accurate friction compensation of a steering system is carried out, thereby achieving an electric power steering apparatus having a good feel of steering.
An electric power steering apparatus according to the second invention, being provided with a torque detector for detecting a steering torque applied to a steering wheel and a steering assist motor which is driven under control based on a drive current corresponding to the steering torque detected by the torque detector, is characterized by comprising: a current detecting circuit for detecting a current flowing in the motor; a voltage detecting circuit for detecting a voltage applied to the motor; motor rotational speed calculating means for calculating a rotational speed of the motor based on the current detected by the current detecting circuit and the voltage detected by the voltage detecting circuit; angular acceleration calculating means for calculating an angular acceleration of turn of the steering wheel by adding up a value corresponding to first-order differentiation of the rotational speed calculated by the motor rotational speed calculating means and a value corresponding to second-order differentiation of the steering torque; and adding means for adding a current determined in relation to the angular acceleration of turn calculated by the angular acceleration calculating means to the drive current.
In this electric power steering apparatus, the torque detector detects a steering torque applied to a steering wheel, and driving of the steering assist motor is controlled based on a drive current corresponding to the detected steering torque. The current detecting circuit detects a current flowing in the motor, and the voltage detecting circuit detects a voltage applied to the motor. The motor rotational speed calculating means calculates a rotational speed of the motor based on the current detected by the current detecting circuit and the voltage detected by the voltage detecting circuit, and the angular acceleration calculating means calculates angular acceleration of turn of the steering wheel by adding up a value corresponding to first-order differentiation of the rotational speed calculated by the motor rotational speed calculating means and a value corresponding to second-order differentiation of the steering torque. The adding means adds a current determined in relation to the angular acceleration of turn calculated by the angular acceleration calculating means to the drive current.
Accordingly, it is possible to achieve an electric power steering apparatus which requires no rotation detector and enables a reduction in the cost of parts.
An electric power steering apparatus according to the third invention, being provided with a torque detector for detecting a steering torque applied to a steering wheel and a steering assist motor which is driven under control based on a drive current corresponding to the steering torque detected by the torque detector, is characterized by comprising: a current detecting circuit for detecting a current flowing in the motor; a voltage detecting circuit for detecting a voltage applied to the motor; motor rotational speed calculating means for calculating a rotational speed of the motor based on the current detected by the current detecting circuit and the voltage detected by the voltage detecting circuit; angular velocity calculating means for calculating an angular velocity of turn of the steering wheel by adding up a value related to the rotational speed calculated by the motor rotational speed calculating means and a value related to first-order differentiation of the steering torque; angular acceleration calculating means for calculating an angular acceleration of turn of the steering wheel from the angular velocity of turn calculated by the angular velocity calculating means; and adding means for adding a current related to the angular velocity of turn and a current related to the angular acceleration of turn calculated by the angular acceleration calculating means to the drive current.
In this electric power steering apparatus, the torque detector detects a steering torque applied to the steering wheel, and driving of the steering assist motor is controlled based on a drive current corresponding to the detected steering torque. The current detecting circuit detects a current flowing in the motor, and the voltage detecting circuit detects a voltage applied to the motor. The motor rotational speed calculating means calculates a rotational speed of the motor based on the current detected by the current detecting circuit and the voltage detected by the voltage detecting circuit, and the angular velocity calculating means calculates an angular velocity of turn of the steering wheel by adding up a value related to the rotational speed calculated by the motor rotational speed calculating means and a value related to first-order differentiation of the steering torque. The angular acceleration calculating means calculates angular acceleration of turn of the steering wheel from the angular velocity of turn calculated by the angular velocity calculating means, and the adding means adds a current related to the angular velocity of turn and a current related to the angular acceleration of turn calculated by the angular acceleration calculating means to the drive current.
Accordingly, friction compensation is performed at the start of steering and more accurate friction compensation of a steering system is carried out, thereby achieving an electric power steering apparatus having a good feel of steering. Moreover, it is possible to achieve an electric power steering apparatus having a good feel of steering by only changing software, without changing conventional hardware.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.